Immunodeficiency of the elderly.

The elderly are at risk for an increased incidence and severity of certain infections. The contribution of age-related immunologic impairment to the pathogenesis of these infections has been difficult to determine because of a number of confounding variables associated with aging. Nevertheless, studies in vitro and in animals support the hypothesis that immunodeficiency accompanies the aging process. Multiple factors may be responsible for altered cell-mediated immunity in the elderly, including thymic involution, reduced levels of thymic hormones, and an increase in the number of immature T lymphocytes. While studies of T cell subpopulations have yielded conflicting results, it appears that T cell proliferative responses are diminished. Aging is also associated with abnormalities of humoral immunity. Although the number and functional activities of neutrophils from healthy elderly persons are relatively intact, diminished bactericidal activity and altered oxygen metabolism have been reported in extremely old individuals. While the relative importance and clinical impact of these immunologic abnormalities remain unclear, future studies may provide new strategies for the prevention and treatment of infections in this rapidly growing segment of the population.